This invention relates to provision of the phase angle of voltage on AC mains for use in a conventional pulse width modulation-controlled voltage source converter employing an insulated gate bipolar transistor (IGBT) bridge, without measuring the voltage of the mains, utilizing signals indicative of the currents in the mains.
Regenerative drives in elevators are attractive since they can transfer energy stored in an overrunning elevator (the elevator load driving the motor) back to the building power supply, resulting in increased energy efficiency and reduced operating costs for the building owner. Proper operation of a regenerative drive requires knowledge of the phase angle of the AC power supply in order to control the energy flow to/from the power supply. Historically, the phase angle has been determined by measuring each of the three power supply phase voltages using a sensing circuit that is typically comprised of high-voltage resistors and isolation amplifiers. The resulting signals are then sent to a microprocessor where the phase angle is calculated.
There are several disadvantages to this approach for determining the phase angle. System cost is increased due to the additional required components, particularly due to the isolation amplifiers that are expensive. Component size is increased due to additional space required on printed circuit boards, which is significant when complying with the regulatory code spacing requirements for high-voltage. Reliability is decreased due to the increased number of components, especially due to high-voltage resistors that historically exhibit manufacturing reliability deficiencies. Complexity is increased due to additional wiring required in the drive from the AC power supply to the printed circuit board, which is prone to manufacturing error.
Objects of the invention include: improved voltage source converters; determining the phase angle of a power source without using voltage sensors; elimination of components and reduced system cost; reduced component size and smaller printed circuit boards; elimination of high voltage spacing requirements; increased reliability and elimination of components that are historically prone to manufacturing reliability deficiencies; and simplified design, reduced manufacturing error and reduction of wiring in voltage source converters.
According to the invention, the phase angle of an AC power source is determined without measuring the AC power source phase voltages, thus eliminating voltage sensing circuits. A short initialization is performed periodically, such as at the beginning of every elevator run, in which the regenerative converter of the drive is energized and de-energized for very short periods of time, during which the current flow through each of the three input phases to the drive are monitored; these current measurements are also used for conventional current feedback in control of the converter. The pulses continue for at least one full cycle of the AC power supply, typically on the order of 16-20 milliseconds. A mathematical algorithm calculates the phase angle of the AC line, which is used to initialize a conventional phase locked loop, which tracks the phase of the line, in the usual fashion, in subsequent operation of the converter; a voltage derived from the commanded in-phase voltage in synchronous coordinates (used to generate the ultimate voltage commands) provides the voltage input to the phase-locked loop.
Although the invention is described with respect to a drive for an elevator system as an exemplary embodiment, the invention may be utilized in drives for any other applicable utilization.
Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing.